An electroluminescent (EL) device is a self-light-emitting device with the advantages of providing a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials for forming a light-emitting layer (see Appl. Phys. Lett. 51, 913, 1987).
An organic EL device changes electric energy into light by the application of electric current to an organic light-emitting material, and commonly comprises an anode, a cathode, and an organic layer formed between the two electrodes. The organic layer of the organic EL device may be composed of a hole injection layer (HIL), a hole transport layer (HTL), an electron blocking layer (EBL), a light-emitting layer (EML) (containing host and dopant materials), an electron buffer layer, a hole blocking layer (HBL), an electron transport layer (ETL), an electron injection layer (EIL), etc.; the materials used in the organic layer can be classified into a hole injection material, a hole transport material, an electron blocking material, a light-emitting material, an electron buffer material, a hole blocking material, an electron transport material, an electron injection material, etc., depending on functions. In the organic EL device, holes from an anode and electrons from a cathode are injected into a light-emitting layer by the application of electric voltage, and an exciton having high energy is produced by the recombination of holes and electrons. The organic light-emitting compound moves into an excited state by the energy and emits light from energy when the organic light-emitting compound returns to the ground state from the excited state.
The most important factor determining luminous efficiency in an organic EL device is light-emitting materials. The light-emitting materials are required to have the following features: high quantum efficiency, high movement degree of an electron and a hole, and formability of a uniform and stable layer. The light-emitting materials are classified into blue light-emitting materials, green light-emitting materials, and red light-emitting materials according to the light-emitting color, and further include yellow light-emitting materials or orange light-emitting materials. Furthermore, the light-emitting material is classified into a host material and a dopant material in a functional aspect. Recently, an urgent task is the development of an organic EL device having high efficiency and long lifespan. In particular, the development of highly excellent light-emitting material compared to conventional light-emitting materials is urgently required considering the EL properties necessary for medium- and large-sized OLED panels. For this, preferably, as a solvent in a solid state and an energy transmitter, a host material should have high purity and a suitable molecular weight in order to be deposited under vacuum. Furthermore, a host material is required to have high glass transition temperature and pyrolysis temperature for guaranteeing thermal stability, high electrochemical stability for long lifespan, easy formability of an amorphous thin film, good adhesion with adjacent layers, and no movement between layers.
Until now, Iridium(III) complexes have been widely known as phosphorescent materials, including bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3), and bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (Firpic) as red, green, and blue light-emitting materials, respectively.
A mixed system of dopant/host materials can be used as light-emitting materials to improve color purity, luminous efficiency, and stability. If the dopant/host material system is used, the selection of the host materials is important since the host materials greatly influence the efficiency and performance of a light-emitting device. In conventional technique, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known phosphorescent host material. Pioneer (Japan) et al., currently developed a high performance organic EL device by employing bathocuproine (BCP), aluminum(III) bis(2-methyl-8-quinolinato)(4-phenylphenolate) (BAlq), etc., which were used in a hole blocking layer, as host materials.
Although these phosphorescent host materials provide good light-emitting characteristics, they have the following disadvantages: (1) Due to their low glass transition temperatures and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum. (2) The power efficiency of an organic EL device is given by [(π/voltage)×current efficiency], and the power efficiency is inversely proportional to voltage. An organic EL device comprising phosphorescent host materials provides higher current efficiency (cd/A) and has a higher driving voltage than one comprising fluorescent host materials. Thus, the organic EL device using conventional phosphorescent host materials has no advantage in terms of power efficiency (Im/W). (3) Furthermore, the operating lifespan and luminous efficiency of the organic EL device are not satisfactory.
Thus, in order to embody excellent properties of the organic EL device, materials constituting the organic layers in the device, in particular host or dopant materials constituting a light-emitting material, should be suitably selected. In this regard, Korean Patent Application Laying-open Nos. 10-2012-0087935 and 10-2012-0095997 disclose fused heterocyclic compounds used as a matrix material, a hole transport or an electron blocking material, an exciton blocking material, or an electron transport or a hole blocking material of a phosphorescent OLED. In addition, U.S. Patent Application Laying-open No. 2011-0303901 discloses indole-quinoline derivatives used as a host or dopant material, a hole transport material, an electron transport material, a hole blocking material, an electron blocking material, a hole injection material, or an electron injection material. However, the organic EL devices comprising the compounds recited in the above publications still do not satisfy power efficiency, luminous efficiency, lifespan, etc. Thus, the present inventors have tried to find organic electroluminescent compounds that can provide an organic EL device with properties superior to the compounds recited in the above publications and have found compounds providing a device with high luminous efficiency and excellent device properties.